1. Field of the Invention
The present invention relates to a sheet material discrimination apparatus for allowing an impact force applying member to collide with a sheet material and detecting an impact force through a sheet material by a detecting unit such as a piezoelectric element, and more particularly, to the improvement of stability of an output of a piezoelectric element to enhance a discrimination accuracy.
2. Related Background Art
In an image forming apparatus adopting an electrophotographic system, an ink jet system, or any other type of printing system, it is preferable to automatically discriminate a sheet material to be processed and adjust an image forming condition, a treating condition, a conveying speed, and the like. As a result, various sheet material discrimination apparatuses for automatically discriminating a sheet material have been proposed.
Japanese Patent Application Laid-open No. 2004-026486 discloses a sheet material discrimination apparatus for detecting an impact force caused when an impact force applying member is allowed to collide with a sheet material by a piezoelectric element. In this case, a sheet material is discriminated by detecting a voltage output generated when the piezoelectric element receives an impact force to be deformed by bending.
Japanese Patent Application Laid-open No. 2005-024550 discloses a sheet material discrimination apparatus for allowing an impact force applying member to collide with a sheet material to detect an impact force through a sheet material by a piezoelectric element. In this case, the piezoelectric element is sandwiched between an impact force receiving member and a cushioning material, and a compressive force due to the impact force received by the impact force receiving member through the sheet material affects on a whole surface of the piezoelectric element. The cushioning material absorbs the impact force received by the piezoelectric element to prevent noise and vibration of a casing.
U.S. Pat. No. 6,397,021 discloses an image forming apparatus for forming an image on a sheet material. In this case, an image is formed in an image forming part, and then temperature of a heat roller of a fixing device for fixing a transferred toner image on a sheet material is detected to control the temperature of the heat roller based on the detected temperature.
In the sheet material discrimination apparatus disclosed by Japanese Patent Application Laid-open No. 2004-026486, a piezoelectric element is positively deformed by bending due to an impact force of an impact force applying member to obtain a large output. However, the piezoelectric element is allowed to be arbitrarily deformed by bending, whereby the piezoelectric element may cause a bending vibration due to an impact force and a contact with the sheet material. Due to the bending vibration of the piezoelectric element, a complicated vibration mode is formed in a plane of the piezoelectric element, to thereby generate a large spike noise which is superimposed on the output of the piezoelectric element. As a result, in a simple output processing for simply detecting peak values and sorting the obtained values, a measurement result high in reproducibility is not be obtained, and a resolving ability for discriminating a sheet material is lowered.
In the sheet material discrimination apparatus disclosed by Japanese Patent Application Laid-open No. 2005-024550, because bending and deformation are constrained by an impact force receiving member, a bending vibration of the piezoelectric element is less likely to be caused. However, when a height of a detecting part, which includes a cushioning material by reducing a width of the impact force receiving member, is to be reduced, a bending rigidity of the piezoelectric element becomes insufficient, whereby a vibration is more likely to be caused. An output of the piezoelectric element becomes a value corresponding to a combined stress for every deformation such as a slip, shearing, compression, and bending of the piezoelectric element. Therefore, in view of extracting a signal component with a good SN ratio in a single mode, it is considered that a component of another mode is included as a noise component.
Therefore, by focusing on a compressing transformation of a stress of a piezoelectric element, the present invention has been made with a structure in which a detecting unit for extracting only a compressional component is provided, and a stable output with less noises is obtained.
Extracting only a compressional component according to the present invention includes not only a case of extracting only compressional components (100% compressional component) but also a case where a compressional component is predominant among the extracted.